This invention relates generally to the deposition of heat or materials onto a substrate and, more particularly, to the use of constrained stagnation flow geometry, including axisymmetric flow, to achieve efficient uniform deposition.
It is well known to those skilled in the art that certain flow configurations have important similarity properties that render their analysis one-dimensional. Included in this set is stagnation flow. Given that a uniform velocity, uniform temperature and uniform composition inlet flow issues from a manifold a fixed distance above a parallel fixed solid surface which is at uniform temperature, it can be shown that the heat and mass flux to the solid surface will be everywhere uniform regardless of the radial extent of the system. In addition, the gas phase species and temperature profiles are independent of radius. The inherent radial uniformity of a stagnation flow geometry provides an important means for achieving uniform species and heat fluxes to large surface areas. This technology offers a means to uniformly clean and etch surfaces and has application to materials synthesis, such as chemical vapor deposition for the fabrication of semiconductors and flame synthesis of diamond films all of which require very highly uniform film growth over relatively large areas so that many identical devices can be cut from a single large wafer.
Various methods for introducing and distributing reactant gases as well as use of specialized geometries, such as rotating-disk and fixed-pedestal reactors, have been designed to try to achieve the desired deposition uniformity. The need for both a method of vapor deposition in which the growth rate of the deposited material onto a substrate is highly uniform over the entire area of the substrate and in which the growth rate of the deposited material can be increased as well as the use of stagnation flow as a means for improving chemical vapor deposition of materials and a method for providing uniform gas flow has been disclosed by deBoer, et al. in U.S. Pat. No. 4,798,165. In this instance, the gas carrying deposition materials is constrained to have an axial symmetry by introducing it into the depostion chamber by means of a multiplicity of apertures. In U.S. Pat. No. 5,215,788 Murayama, et al., disclosed that a very uniform deposit could be produced at a growth rate of 60 microns/hr in the chemical vapor depostion synthesis of diamond by the use of a highly strained premixed flat flame stabilized in the stagnation flow regime. While the outermost gas flow is crucial to maintaining the ideal streamlines necessary for stagnation flow, this gas flow does not contribute to development of the deposit. As described above, stagnation flow offers numerous advantages insofar as a means for improving uniformity of distribution of reactants over large area substrates, however, in order for this technique to become practical the inefficiencies in the use of reactants must be overcome.
In addition to the fields of cleaning and etching of surfaces, chemical vapor deposition and material synthesis with flames, the use of strained stagnation flow provides a new route to combustion devices that are energy efficient, in the sense that they are effective in coupling flame generated heat to surrounding surfaces and working media, and offer a means of minimizing emissions by controlling the gas phase combustion process.
In order to achieve energy efficiency in combustion applications effective exchange of heat between the flame gases and the working medium is important, especially for natural gas flames where heat extraction is heavily dependent upon convective heat transfer rather than direct radiation. Because gas inlet velocities can be very high, stagnation flames offer a very effective route to increased heat transfer and, consequently, greater energy efficiency. Fukushima, et al. have used the stagnation flame approach in a steel making application achieving a surface heat flux of approximately 200 kW/m.sup.2, almost five times greater than that provided by electric powered radiant tubes.
Because strained stagnation flow permits high gas inlet velocities the flame can be driven very near a heat transfer surface. Since gas velocities can be high, residence times will be correspondingly low and, as a consequence, emissions of NO.sub.x from stagnation flames are low. In addition, as discussed above, surface heat transfer rates can be very high and, consequently, maximum flame temperatures can be reduced, further reducing NO.sub.x emissions. However, as is the case with other applications of stagnation flow, vide supra, gas flow that enters the system beyond a critical radius does not contribute to the combustion process and is, in that sense, wasted.
It is obvious to those skilled in the art, that stagnation flow systems offer both significant advantages in combustion and materials processing and synthesis applications. The only remaining impediment to widespread use of stagnation flow systems is the need to make more efficient use of reactants. Maintaining the desirable properties of stagnation flow coupled with a practical solution to the problem of a more efficient stagnation flow system forms the basis of the invention disclosed herein.